o que é este erro?

Olá galerinha, beleza?Estou tentando entender um script de carrinho de compras que pequei aqui neste site. Funciona, mas aparecem estas mensagens estranhas. O que é isto e como resolvo o problema?Notice: Use of undefined constant CODIGO - assumed 'CODIGO' in c:\inetpub\wwwroot\php\carrinho\index.php on line 13Notice: Use of undefined constant ARTISTA - assumed 'ARTISTA' in c:\inetpub\wwwroot\php\carrinho\index.php on line 14Notice: Use of undefined constant ALBUM - assumed 'ALBUM' in c:\inetpub\wwwroot\php\carrinho\index.php on line 15Notice: Use of undefined constant PRECO - assumed 'PRECO' in c:\inetpub\wwwroot\php\carrinho\index.php on line 16Notice: Use of undefined constant IMAGEM - assumed 'IMAGEM' in c:\inetpub\wwwroot\php\carrinho\index.php on line 17Notice: Use of undefined constant CODIGO - assumed 'CODIGO' in c:\inetpub\wwwroot\php\carrinho\index.php on line 19Notice: Use of undefined constant ARTISTA - assumed 'ARTISTA' in c:\inetpub\wwwroot\php\carrinho\index.php on line 20Notice: Use of undefined constant ALBUM - assumed 'ALBUM' in c:\inetpub\wwwroot\php\carrinho\index.php on line 21Notice: Use of undefined constant PRECO - assumed 'PRECO' in c:\inetpub\wwwroot\php\carrinho\index.php on line 22